Method for manufacturing micromechanical components

ABSTRACT

The present invention relates to a method for manufacturing an acceleration sensor. In the method, thin SOI-wafer structures are used, in which grooves are etched, the walls of which are oxidized. A thick layer of electrode material, covering all other material, is grown on top of the structures, after which the surface is ground and polished chemo-mechanically, thin release holes are etched in the structure, structural patterns are formed, and finally etching using a hydrofluoric acid solution is performed to release the structures intended to move and to open a capacitive gap.

This application is a Divisional of co-pending U.S. application Ser. No.11/920,687 filed on Feb. 25, 2008, and for which priority is claimedunder 35 U.S.C. §120, whereby U.S. application Ser. No. 11/920,687 is aNational Stage Entry under 35 U.S.C. §371 of PCT Application No.PCT/FI2006/000174 filed on Jun. 2, 2006, which claims foreign priorityto Application No. 20050592 filed in Finland on Jun. 3, 2005, the entirecontents of all being hereby incorporated by reference.

The present invention relates to a method for implementing narrow gapsin micromechanical components, in a micromechanical SOI-wafer structure.

The invention also relates to a silicon micromechanical structure andthe use of a silicon micromechanical structure.

In the method according to the prior art, silicon-micromechanicaldevices, such as acceleration sensors and other similar devices, aremainly manufactured by using SOI (silicon-on-insulator) waferstructures, on which grooves are etched, the walls of which are oxidizedand the surface of which is then planarized by fluorine-plasma etching.

In a publication by Hsu et al. [W.-T. Hsu, J. R. Clark, and C. T.-CNguyen, ‘A sub-micron capacitive gap process formultiple-metal-electrode lateral micromechanical resonators’. Proc. IEEMEMS Conference, (Interlaken Switzerland, 2001), p. 349] a method isdisclosed for forming horizontal capacitive gaps between amicromechanical structure and its metal electrodes, in which methodmembranes of oxide and LPCVD silicon nitride are grown, which layersinsulate the devices and the connector units from an electricallyconductive silicon substrate, after which a layer ofjunction-polysilicon is formed, which is doped and on which structuralpatterns are formed. An oxide layer is again formed on top of thestructural patterns, which acts as a sacrificing layer in later plasmaetching. After forming vias, a layer of structural polysilicon isformed, which is doped and heat-treated. The final structure is achievedby plasma etching. According to the method of the publication, theelectrodes in the finished structure are metal, whereas the moving partsare polysilicon.

Vertical capacitive gaps too have been formed on an SOI substrate [S.Pourkamali and F. Ayazi, ‘SOI-Based HF and VHF Single-Crystal SiliconResonators with Sub-100 Nanometer Vertical Capacitive Gaps,’ Digest ofthe 12^(th) International Conference on Solid State Sensors, Actuatorsand Microsystems (Transducers '03), Boston, Jun. 8-12, 2003, pp.837-840], using a method that corresponds to that disclosed in Hsu etal.'s publication. As in the previous publication, in the method of thispublication too the final structure is achieved using plasma etching. Inthe structures presented in the publication, the electrodes are ofpolysilicon while the moving parts are of single-crystal silicon.

Quey et al. have shown in a publication (E. Quevy, B. Legrand, DCollard, L. Buchaillot, ‘Ultimate Technology for Micromachining ofNanometric Gap HF Micromechanical Resonators’. 16^(th) IEEE MicroElectro Mechanical System, Kyoto, Japan, 2003) that horizontalcapacitive gaps can be made as narrow as 60 nm. According to thepublication in question, two materials are used in the moving parts,i.e. single-crystal silicon and LPCVD polysilicon typically grown fromdisilane. Strongly doped polysilicon was used as the material of theelectrodes.

A manufacturing method for a micromechanical resonator is disclosed in apublication by Kaajakari et al. (Ville Kaajakari, Tomi Mattila, AarneOja, Jyrki Kiihamäki, and Heikki Seppä, Square-Extensional ModeSingle-Crystal Silicon Micromechanical Resonator for Low-Phase-NoiseOscillator Applications, IEEE Electron Device Letters, Vol. 25, No. 4,2004).

Even though structures, which have an even topography and in which thereare even narrow capacitive gaps, have been created with the aid of theprior art, these solutions have, however, the drawback that the surfaceof the structures remains rough from using plasma etching, whereby thestructures in question cannot be closely attached to other structures,as air and particles can enter the interior parts of the structures.

In addition, a so-called planarizing resistance must be used in planaretching. The thickness of this resistance, together with the thicknessof the LPCVD polysilicon, limits the thickness of the availablestructural layer.

There is a need for a method, by means of which structures with even,smooth surfaces can be created, which can be tightly connected to othersimilar structures and to the walls of devices, using a manner ofevening the surfaces, which will not limit the thickness of thestructural layer.

The present invention is intended to provide an improved method formanufacturing silicon-micromechanical structures, such as resonators,acceleration sensors, and other similar devices. By means of a preferredembodiment of the invention, grinding and polishing can be used tocreate structures with smooth surfaces.

By means of the present invention, micromechanical components forimplementing narrow gaps are manufactured from SOI-wafer structures, inwhich structures grooves are etched, the walls of which are oxidized.According to the method:

-   -   a) a thick layer of an electrode material is grown, which layer        covers all other material,    -   b) the surface is smoothed by grinding,    -   c) the silicon is polished using chemo-mechanical polishing,        whereby the height of the surface is also returned to its        original level,    -   d) release holes are etched into the structure,    -   e) structural patterns are created, and    -   f) etching using hydrofluoric acid is performed to release the        structures intended to move.

More specifically, the method according to the invention implementsnarrow gaps in micromechanical components from a micromechanicalSOI-wafer structure, in which grooves are etched, the walls of which areoxidized, the method comprising a) growing a thick layer of electrodematerial (14) on the wafer structure, which layer covers all othermaterial, b) evening the surface by grinding, c) polishing the surfaceby chemo-mechanical polishing, d) etching thin release holes (7) intothe structure, e) forming structural patterns (1, 8, 9), and f) etchingwith a hydrofluoric acid solution to release the structures (1, 8, 9).

The silicon-micromechanical structure according to the present inventionis a structure which comprises a body (12), a mass (1) attached flexiblyto the body (12), a capacitor structure (8, 9) formed between the body(12) and the mass (1), wherein protrusions (8, 9) corresponding to eachother and facing each other are formed in both the mass (1) and the body(12), in order to create the greatest possible capacitive density.

The silicon-micromechanical structure according to the invention can beused, for example, in wafer-bonding applications, in vacuum-sealing orpackaging, or in applications in which a large capacitance is requiredbetween the electrodes.

The method according to the invention differs from the prior artparticularly in the stage of smoothing the surface, in which, in themethod of the present invention, plasma etching as in the referencepublications is not used, but instead mechanical grinding andchemo-mechanical polishing.

In a preferable embodiment of the present invention, also a comb-likeelectrode structure can be formed.

An advantage of the method of one embodiment of the invention is thatwith the aid of grinding and polishing a very even and smooth surface ispresented, which on the other hand makes it possible to manufacture e.g.air tight structures and devices using different wafer-bondingtechniques.

An advantage of a second embodiment of the method is that it is alsosuitable for use with a greater structural thickness than that inmethods according to the prior art, because in it there is no need of aplanarizing resistance that limits thickness.

Using an embodiment of the present invention, in which the surface isfirst smoothed mechanically by grinding and then polished usingchemo-mechanical polishing (CMP), the structure is made more even andsmooth than by means of the plasma etching used in the prior art.

Other details and advantages of the invention will become apparent fromthe following detailed description.

FIG. 1 a is a detailed view of a 1-D acceleration sensor according toone preferred embodiment of the invention. The view from above providesa general view of the entire structure.

FIG. 1 b is a cross-section (B-B′) of the structure in FIG. 1 a.

FIG. 1 c is a close-up view of the protrusion structure.

FIG. 1 d is a cross-section (A′-A) of the protrusion structure.

FIG. 2 shows the manufacturing method of the silicon-micromechanicalcomponents.

FIG. 3 a shows a top view of a flat resonator according to a preferredembodiment of the invention.

FIG. 3 b is a cross-section of one alternative solution to the structurein FIG. 3 a.

FIG. 3 c is a cross-section of a preferred embodiment of the structurein FIG. 3 a.

The following numbering is used in the figures:

-   -   1 floating mass of single-crystal silicon    -   2 groove travelling through the substrate    -   3 electrodes    -   4 anchor    -   5 suspension spring    -   6 narrow gap    -   7 hole for release etching    -   8 protrusion structure of electrodes    -   9 protrusion structure of single-crystal silicon    -   10 recess in the SOI-wafer structure    -   11 longitudinal groove between the protrusion structures    -   12 substrate    -   13 transverse groove between the protrusion structures    -   14 electrode material

In FIG. 1, a moving structure 1 made of single-crystal silicon ‘floats’on four anchors 4, to which it is attached with the aid of suspensionsprings 5. A groove 2, which ensures the movability of the structure bykeeping its moving structure 1 separate from the substrate 12, runsaround the moving structure 1. Extra holes 7 are also made in thesingle-crystal silicon for the release etching. In the single-crystalsilicon there are at least two protrusions 9 while opposite to them areset a corresponding number of electrode protrusions 8. A narrow gap 6,which consists of longitudinal grooves 11 and transverse grooves 13, andwhich forms the air gap of the junction capacitor formed by theprotrusions 8, 9, is etched between the protrusions 9 of thesingle-crystal silicon and the protrusions 8 of the electrodes. Theprotrusions give the electrode structure a comb-like appearance whenviewed from above.

FIG. 2 shows the various stages (Stages 1-7) of the manufacturing methodof the comb-like electrode structures advantageously formed in thesilicon-micromechanical components:

-   -   1) a recess 10 is etched into the SOI-wafer structure,    -   2) both longitudinal 11 and transverse 13 grooves are etched        into the wafer at the location of the recess 10,    -   3) the walls of the grooves 11, 13 are oxidized,    -   4) a thick layer of electrode material 14 is grown on the entire        structure,    -   5) the surface is evened by grinding and chemo-mechanical        polishing,    -   6) structural patterns and release holes 7 are formed, and    -   7) etching with a hydrofluoric acid solution is performed to        release the structures.

In the flat resonator according to a preferred embodiment of theinvention, shown in FIG. 3, a fixed silicon area is anchored to thesubstrate 12, with the aid of anchors 4. In the structure of the flatresonator, there is a similar ‘floating’ silicon plate 1 to that in theacceleration sensor shown in FIG. 1. The cross-sectional alternativesshow electrode structures according to preferred embodiments of theinvention, of which one is unified (FIG. 3 b) and in another there arethin protrusions (FIG. 3 c). In both alternatives, the resonator 1 isseparated from the electrode structure 3 by only a narrow gap 6.

The term ‘SOI’, used in connection with the invention, refers to siliconon top of an insulating material (silicon-on-insulator). SOI substrates,i.e. SOI wafers, are used particularly in the manufacture of smallstructures requiring great precision, in which they act as anelectrically insulating base material. The greatest area of applicationfor SOI wafers are high-speed, tightly integrated circuits. In these,the SOI structural layers used are thin (hundreds of nanometres),whereas in micromechanics there are several or tens of micrometres.

‘Polysilicon’ is multi-crystalline silicon. In connection with thepresent invention, it is used mainly as an electrode material. In themethod according to the invention, a second material that is much usedis single-crystal silicon, which is a mechanically stable material,whereby it is highly suitable for purposes according to the presentinvention. Both polysilicon and single-crystal silicon are widely usedin the manufacture of micromechanical devices.

According to one preferred embodiment of the present invention, asilicon-micromechanical structure is manufactured from thin SOI-waferstructures, in which grooves are etched (FIG. 2, Stages 1-2). Thethickness of the wafer is about 5-150 μm, preferably about 20 μm.Thermal oxidation is used to grow a thin oxide membrane on the walls ofthe grooves (Stage 3), which when etched in a later stage acts as asacrificing layer and the thickness of which determines the width of thecapacitive gap in the final structure. The thickness of the membrane canvary, but it is generally 50 nm, or thinner/thicker, for example, 10-200nm. After this, the grooves are filled with a thick layer of electrodematerial (Stage 4), which layer covers all the other material.

After the growing of the electrode material, the surface remains uneven,because it conforms to the original shape of the structure. The surfacemust therefore be evened. Evening is performed in the preferredembodiments of the invention by first of all rough mechanical grindingand then by chemo-mechanical polishing (CMP)(Stage 5). After polishing,the surface is smooth and its height has returned to the original level,whereby among other things the oxide layer to be used as the sacrificinglayer appears from beneath the electrode material (polysilicon).

After the surface has been evened, narrow or small holes are formedthrough the structural layer, which holes facilitate the etchingsolution used later in the release etching to penetrate to the variousparts of the structure, and structural patterns are formed in thesingle-crystal structural silicon (Stage 6). The diameters of therelease holes are about 1-10 μm, preferably about 4 μm.

The final structure is created by etching (Stage 7), in which an HFsolution (hydrofluoric acid solution) is used as the etching solution,whereby the structures that are intended to move are released and, amongother things, the thin silicon-oxide film between the electrodes and themoving parts is removed. A very narrow capacitive gap remains in placeof the said oxide film.

According to a second preferred embodiment of the present invention, theSOI wafer used for the manufacture of the silicon-micromechanicalstructures is first of all etched with a recess 10, in which grooves arethen etched, of which grooves some 11 are longitudinal relative to thedirection of viewing FIG. 1 d and the others 13 are transverse relativeto the direction of viewing FIG. 1 d. In this preferred embodiment, thetransverse grooves 13 form a comb-like structure, which comprises atleast two, preferably 3-4, protrusions 9 of single-crystal silicon. Dueto the recess 10, the height of this structure is about 1-2 μm,preferably about 1 μm, lower than the height of the structuresurrounding it. Thermic oxidation is used to grow an oxide film of thewalls of the silicon-finger-comb structure, which when etched in a laterstage acts as a sacrificing layer. After this, the grooves between thesilicon protrusions 9 are filled with a thick layer of electrodematerial 14 (polysilicon), which layer covers all other material.Typically there are the same number of polysilicon protrusions 8 asthere are of single-crystal silicon protrusions 9 (see FIG. 1 a). Ontheir side, there can also be 1 or 2 fewer polysilicon protrusions 8than single-crystal silicon protrusions 9. As the electrode material 14fills not only the grooves, but also the recess 10 previously etchedinto the wafer, the polysilicon fingers 8 of the electrodes remain incontact with each other, even after the evening of the surface, in whichcase the heights of the other structures are returned to their originallevel, revealing the oxide layer grown on top of them (FIG. 2, stage 5).Also according to this embodiment, the evening of the surface isachieved by grinding and polishing, as a result of which an even andsmooth surface is obtained, which remains even and smooth in the endproduct (FIG. 2, Stage 7).

By means of this second preferred embodiment, comb-like electrodestructures 3 are obtained, which create a larger capacitor surface areaand a larger total capacitance between the moving structures 1 and thefixed body 12. The narrow gaps permit the use of a lower operatingvoltage when the micromechanical component is controlledelectrostatically.

Polysilicon, e.g., LPCVD polysilicon, preferably epipolysilicon grown inan epireactor, can be used as the electrode material. Single-crystalsilicon is preferably used in the moving parts 1. The electrodes 3 arefixed and are separated from the fixed structural silicon by only theaforementioned narrow gap 6.

The silicon protrusions 8 in the electrode structure according to theaforementioned second embodiment (FIG. 2) form a comb-like structure inthe longitudinal direction. Due to the recess 10 previously made in theSOI wafer structure, the protrusions 8 in the electrode structure areconnected by a unified electrode structure about 1-2-μm thick, whichfills the recess that was previously in this place. The polysiliconprotrusions 8 in question reinforce the structure and prevent theoutermost parts of the electrode structure 3 from wobbling, even thoughthe solution used in the release etching has been partly able topenetrate even under and behind the polysilicon layer.

The widths (W_(<Si>)) of the single-crystal silicon protrusions 9 are atleast a few micros, preferably about 5 μm while the grooves thatseparate them, from which the polysilicon protrusions 8 are formed, areabout 4 μm wide and at right-angles relative to the surface of thewafer. The total width (W_(poly)) of the entire electrode structure 3can be calculated according to the following equation,

W _(poly) =n*4+(n−1)*5

in which n is the number of narrow grooves filled with polysilicon(e.g., in FIG. 2, n=3) while the number 4 corresponds to the width ofthe grooves in question. For example, if the number of grooves is 3(i.e. n=3), as in FIG. 2, the width of the entire comb-like structure,i.e. the electrode structure 3, can, with the aid of the equation, becalculated to be about 27 μm. By varying the widths of thesingle-crystal protrusions 9 and the grooves between them, electrodestructures 3 with a total width of about 20-35 μm can be formed (whenn=3).

In order to release the single-crystal silicon from the base material,i.e. the SOI wafer, the preferred embodiments according to the presentinvention include the etching of release holes 7 in the single-crystalsilicon prior to the final HF etching, which is performed in order torelease the structures. With the aid of the release holes 7, the etchingsolution will be able to act under even large areas of single-crystalsilicon.

The maximum height of the silicon protrusions is determined by the factthat the said release holes 7 in the single-crystal silicon must be madethrough the structural layer. By making larger holes, it is possible touse a thicker structural layer. The ratio (aspect ratio, max AR) of thethickness of the structural layer to the widths of the holes is 10:1,i.e. if the diameter of the hole is about 4 μm, a structural layer witha thickness of 40 μm can be used. AR is to some extent a device-specificnumber.

If the aforementioned release holes are not needed, the maximum heightof the silicon protrusions is determined by the etching and filling ofthe grooves in the comb-like structure. If the estimated max AR of thegrooves is 25:1 and the thickness of the filler layer is 2 μm, thegreatest possible groove depth will be about 100 μm, calculatedaccording to the following equation:

height=(2+2)*max AR

The width of the aforementioned grooves is determined by only thethermic oxidation, by means of which the etched oxide film is laterformed on the walls of these grooves.

As can be seen from FIG. 1, each single-crystal silicon protrusion 9forms a pair with one polysilicon protrusion 8. These pairs 8, 9 formsaid comb-like structure. In FIG. 1 a, there are four of these pairs ateach electrode 3. The total capacitance (C_(tot)) of the electrodestructure according to the figure in question would thus be four timesgreater than the capacitance (C₁) of a structure with only one pair ofprotrusions 8, 9. The total capacitance is thus directly proportional tothe number (n) of protrusions pairs 8, 9.

C _(tot) =n*C ₁

The method according to the present invention and the structures orcomponents manufactured with the aid of the method can be used in manydifferent ways. Typical end products for the process according to theinvention are silicon-micromechanical structures. Such are, for example,silicon-micromechanical flat resonators, which are used as oscillatingcircuits in electronic resonators, in other words as replacement forquartz crystals. Alternatively, the invention can be applied, forexample, to the manufacture of acceleration sensors.

Structures or components manufactured according to the invention can beused in various wafer-bonding applications, in which by using theinvention it is possible to create, for example, structures suitable forvacuum sealing or packaging, since the surface evened by grinding andchemo-mechanical polishing is much more even than a surface evened byetching, whereby air or dirt and other particles cannot enter thesevacuum-sealed structures or packages.

The method according to the present invention also permits themanufacture of micromechanical components and their use in applications,in which a large capacitance is required between the electrodes.Capacitive micromechanical sensors are widely used, for instance, in theautomotive industry. The strengths of microsensors include a low powerconsumption and integrability in electronics. They are completelysuitable, for example, for precision measurements. An accelerationsensor manufactured according to the invention can be used, for example,in automobiles, to measure acceleration, or in various industrialequipment, to measure the level of vibration.

1. Method for implementing narrow gaps in micromechanical componentsfrom a micromechanical SOI-wafer structure, in which grooves are etched,the walls of which are oxidized, said method comprising: a) growing athick layer of electrode material on the wafer structure, which layercovers all other material, b) evening the surface by grinding, c)polishing the surface by chemo-mechanical polishing, d) etching thinrelease holes into the structure, e) forming structural patterns, and f)etching with a hydrofluoric acid solution to release the structures. 2.Method according to claim 1, wherein the used electrode material isepipolysilicon grown in an epireactor.
 3. Method according to claim 1,wherein single-crystal silicon is used in the moving parts of themicromechanical components.
 4. Method according to claim 2, whereinsingle-crystal silicon is used in the moving parts of themicromechanical components.
 5. Method according to claim 1, wherein thegrooves etched into the structure of the wafer form a comb-likestructure comprising at least two protrusions of single-crystal silicon.6. Method according to claim 5, wherein a recess is etched into thewafer structure prior to the etching of the grooves, whereby thecomb-like structure is brought to a level that is about 1 μm lower thanthat of the rest of the structure.
 7. Method according to claim 6,wherein the grooves in the comb-like structure are filled with anelectrode material, in such a way that the material fills completely atleast the grooves and the recess.
 8. Method according to claim 1,wherein hydrofluoric-acid etching is used in order to release thestructures intended to move and in order to create a capacitive gapbetween the moving structures and the electrodes.
 9. Use of the methodaccording to claim 1 in wafer-bonding applications, whereby structuresare created, which can be used in vacuum-sealing or packaging.